[XFS] add keys_inorder and recs_inorder btree methods

Add methods to check whether two keys/records are in the righ order. This
replaces the xfs_btree_check_key and xfs_btree_check_rec methods. For the
callers from xfs_bmap.c just opencode the bmbt-specific asserts.

SGI-PV: 985583

SGI-Modid: xfs-linux-melb:xfs-kern:32208a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Bill O'Donnell <billodo@sgi.com>
Signed-off-by: David Chinner <david@fromorbit.com>